Products utilizing a piezoelectric ceramic include, for example, piezoelectric sensors, filters, piezoelectric resonators, ultrasonic oscillators, ultrasonic motors, and the like.
Piezoelectric sensors are used as shock sensors, acceleration sensor, or on-vehicle knocking sensors. In particular, in recent years, research has been conducted on use of a piezoelectric sensor as a pressure sensor for directly detecting a pressure in a cylinder to optimize a timing of fuel injection from an injector in order to improve fuel efficiency and reduce exhaust gas (HC, NOx).
A mechanism for detecting a change in pressure in a cylinder is now described. The pressure comprises, for example: a pressure transmission pin protruding in a cylinder of an engine; and a piezoelectric sensor which detects a change in pressure transmitted through the pressure transmission pin in the cylinder. A portion of a head of the pressure transmission pin protrudes inside the cylinder so as to transmit pressure in the cylinder, and the portion is subjected to high temperatures during combustion in the cylinder. Consequently, in addition to a large change in pressure, heat is also transmitted to the piezoelectric sensor connected to the pressure transmission pin, and its temperature reaches 150° C.
Existing piezoelectric ceramics comprise, a PZT (lead zirconate titanate)-based material or a PT (lead titanate)-based material, which has a high piezoelectric property and a large piezoelectric constant d.
However, it has been pointed out that, since the PZT- or PT-based material contains about 60% by mass of lead, there is a possibility that leaching of lead may be caused by acid rain, resulting in environmental contamination.
In addition, since the PZT-based material or the PT-based material has a Curie temperature Tc of about 200° C. to 300° C., the piezoelectric constant d decreases when used under a high temperature of about 150° C., and the piezoelectric constant d at 150° C. largely varies with respect to the piezoelectric constant d at room temperature. Therefore, use thereof has been restricted greatly. For example, in the case where a piezoelectric material that is a PZT-based material or PT-based material is used as a pressure sensor which directly detects the pressure in an engine cylinder, when the piezoelectric material is subjected to a high temperature of 150° C., it changes with time, resulting in a decrease in the piezoelectric constant d. Therefore, the output voltage varies even under the same applied pressure, and since the piezoelectric constant d at 150° C. largely varies with respect to the piezoelectric constant d at room temperature, linearity cannot be obtained in the relationship between pressure and output voltage, and it is difficult to calculate an accurate pressure from the output voltage.
On the other hand, in order to obtain pressure sensor characteristics which are stable even under a high temperature of 150° C., use of a single crystal of langasite, quartz, or the like has also been studied. However, in the case of a single crystal, the piezoelectric constant d is small, which is a problem. Moreover, the single crystal is easily chipped and cracked during processing, and is easily cracked when pressure is applied during actual use. Furthermore, the production cost of the single crystal is very high, which is a problem.
Under these circumstances, there is a high expectation for piezoelectric materials not containing lead. As a piezoelectric material not containing lead, for example, Patent Document 1 proposes a material containing, as a main component, a bismuth layered compound. Many piezoelectric ceramics containing, as a main component, a bismuth layered compound have a Curie temperature of about 400° C. or higher. Such piezoelectric materials have high heat resistance, and there is a possibility that they can be applied to piezoelectric elements used in an environment subjected to high temperatures, for example, in an engine room.
However, in the piezoelectric ceramic mainly composed of a bismuth layered compound described in Patent Document 1, the hysteresis of the output charge caused by a positive piezoelectric effect when a load is applied is large. That is, the difference between the output charge caused in a state in which the applied load increases and the output charge caused in a state in which the applied load decreases is large, which is a problem. Accordingly, when such a piezoelectric ceramic is used, for example, as a piezoelectric element for a pressure sensor, the difference in hysteresis causes an error in the load measured, and its pressure detection accuracy is degraded.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-167276